Wallzilla killed the volume.
I'm really curious as to why there's a wall in many currencies.
Gox has only one market for all currencies. The bids/asks in any given currency are converted to show in all other currencies, with a exchange fee built in. The fee does not go to Gox, since they ironically are not licensed to exchange between various fiat currencies. Instead the fee goes to their bank, which does the actual fiat exchange.
The upshot is that if you place a bid in USD it shows up in Pounds, Euros, Baht, whatever else as well.
Source: MagicalTux in irc a few weeks ago.
That's how it works. It's in their FAQ, BTW